With the continuing decrease of technology node, demands for pattern size precision and overlay accuracy are increasing. However, during the lithographic process, due to the resolution limitation of optical imaging, the right-angled pattern corner in the photomask layout will be inevitably subjected to rounding distortion when being exposed finally and imagined on a silicon wafer. If not being corrected properly, such corner distortion will cause many problems like size reduction of the pattern corner, which will bring adverse effects on the overlay accuracy and the pattern coverage rate. More seriously, such rounding distortion may reduce the actual process window in the process for forming patterns which conform the design rules and considered safe. For example, the corner rounding of the patterns in the metal layer as shown in FIG. 1 reduces the coverage area with the contact layer, resulting in the shortage of pattern coverage. To avoid the pattern corner distortion, an Optical Proximity Correction (OPC) is generally adopted to perform correction in the industry. The OPC method for corner rounding in the prior art mainly includes a rule-based OPC using serif correction and a model-based OPC.
In early stage, in the rule-based OPC, corner rounding is usually improved by adding serif patterns. The size of the added serif is generally selected from empirical values, or determined by designing a test pattern and actual measuring wafer data. However, as the pattern size is becoming smaller and smaller, such photomask pattern correction method is not applicable since the Optical Proximity effect becomes more obvious on patterns of small size. Therefore, the size of the serif required to be added at different positions and under various conditions cannot be determined depending on simple experience or limited measurement results, which may easily cause overcompensation or under-compensation as shown in FIG. 2.
Although the model-based OPC seems to be a possible solution since it can be applied under different conditions, the compensation effect at the pattern corner will be affected and limited by the correction segment when the model-based OPC is used. In theory, the correction effect becomes better as the segment is finer, but in practical application, the boundary segment is not generally made fine to ensure a reasonable calculation time and a better global correction result.
In addition, since there is a competitive relation between the corner and the straight line, when the corner distortion is greatly reduced, it may cause the undulate of the straight line and a worse correction result. Therefore, it is still difficult to improve the corner correction by model-based OPC.
At present, there lacks a correction manner which combines the above two methods. The correction result is not satisfied by simply combining the model-based OPC and the serif correction, since the jogs introduced by the serif correction will affect the segment of the target pattern, or even cause a deviation of the corrected pattern from the target pattern. As shown in FIG. 3, a larger serif will directly cause a corner convex to deviate from the target right-angled corner, while a smaller serif may even worsen the rounding of the corner in addition to affects the segment. Therefore, there exists a need to provide a correction manner which can effectively combine the model-based OPC and the serif correction.